1. Field of the Invention
This invention relates to a display device for reconstructing three-dimensional (3D-) scenes using large-area computer-generated video holograms (CGH) encoded in a spatial light modulator which includes electronically controllable cells. Said cells modulate the amplitude and/or phase of light by encoding each cell with hologram values corresponding to a video-hologram. Like auto-stereoscopic displays, reconstructions of video holograms also aim to present moving two- or three-dimensional scenes using a video display.
In this document a CGH describes a hologram that is calculated from a scene. The CGH comprises complex-valued numbers representing the amplitude and phase of light waves that are needed to reconstruct the scene. The CGH may be calculated e.g. by coherent ray tracing, by simulating the interference between light reflected by the scene and a reference wave, or by Fourier or Fresnel transforms.
A spatial light modulator (SLM) modulates a wave front of the incoming light. An ideal SLM would be capable of representing arbitrary complex-valued numbers, i.e. of separately controlling the amplitude and the phase of the light wave. However, a typical SLM controls only one property, either amplitude or phase, with the undesirable side effect of also affecting the other property.
There are different ways to modulate the light in amplitude or phase, e.g. electrically addressed liquid crystal SLM, optically addressed liquid crystal SLM, micro mirror devices or acousto-optic modulators. The modulation of the light may be spatially continuous or composed of individually addressable cells, one-dimensionally or two-dimensionally arranged, binary, multi-level or continuous.
A suitable spatial light modulator for holographic reconstruction of a 3D-scene is for example a Liquid Crystal Display (LCD). However, this invention can also be applied to other controllable spatial light modulators that modulate a light wave front. The invention can also be applied to continuous spatial light modulators, e.g. an optically addressed spatial light modulator (OASLM).
2. Background of the Invention
In the present document, the term “encoding” denotes the way in which the cells of the spatial light modulator are supplied with control values of cells of the video hologram so that a 3D-scene can be reconstructed from it.
In contrast to auto-stereoscopic displays, with video holograms an observer sees an optical reconstruction of a light wave front of a three-dimensional scene.
According to the present invention, the 3D-scene is reconstructed in a space that stretches between the eyes of an observer and the spatial light modulator (SLM). The SLM can also be encoded with video holograms such that the observer sees objects of a reconstructed three-dimensional scene in front of the SLM and other objects on or behind the SLM.
The cells of the spatial light modulator are preferably transmissive cells which are passed through by light, the rays of which are capable of generating interference at least at a defined position and over a coherence length of a few millimeters. This allows holographic reconstruction with an adequate resolution in at least one dimension. This kind of light will be referred to as ‘sufficiently coherent light’.
In order to ensure sufficient temporal coherence, the spectrum of the light emitted by the light source must be limited to an adequately narrow wavelength range, i.e. it must be near-monochromatic. The spectral bandwidth of high-brightness LEDs is sufficiently narrow to ensure temporal coherence for holographic reconstruction. The diffraction angle at the SLM is proportional to the wavelength, which means that only a monochromatic source will lead to a sharp reconstruction of object points. A broadened spectrum will lead to broadened object points and smeared object reconstructions. The spectrum of a laser source can be regarded as monochromatic. The spectral line width of a LED is sufficiently narrow to facilitate good reconstructions.
Spatial coherence relates to the lateral extent of the light source. Conventional light sources, like LEDs or Cold Cathode Fluorescent Lamps, can also meet these requirements if they radiate light through an adequately narrow aperture. Light from a laser source can be regarded as emanating from a point source within diffraction limits and, depending on the modal purity, leads to a sharp reconstruction of the object, i.e. each object point is reconstructed as a point within diffraction limits.
Light from a spatially incoherent source is laterally extended and causes a smearing of the reconstructed object. The amount of smearing is given by the broadened size of an object point reconstructed at a given position. In order to use a spatially incoherent source for hologram reconstruction, a trade-off has to be found between brightness and limiting the lateral extent of the source with an aperture. The smaller the light source, the better is its spatial coherence.
A line light source can be considered to be a point light source if seen from a right angle to its longitudinal extension. Light waves can thus propagate coherently in that direction, but incoherently in all other directions.
In general, a hologram reconstructs a scene holographically by coherent superposition of waves in the horizontal and the vertical directions. Such a video hologram is called a full-parallax hologram. Given a sufficiently large observer window or observer region the reconstructed object facilitates motion parallax in the horizontal and the vertical directions, like a real object. However, a large observer region requires high resolution in both the horizontal and the vertical direction of the SLM.
Often, the requirements on the SLM are lessened by restriction to a horizontal-parallax-only (HPO) hologram. The holographic reconstruction takes place only in the horizontal direction, whereas there is no holographic reconstruction in the vertical direction. This results in a reconstructed object with horizontal motion parallax. The perspective view does not change upon vertical motion. A HPO hologram requires less resolution of the SLM in vertical direction than a full-parallax hologram. A vertical-parallax-only (VPO) hologram is also possible but uncommon.
The holographic reconstruction occurs only in the vertical direction and results in a reconstructed object with vertical motion parallax. There is no motion parallax in the horizontal direction. The different perspective views for the left eye and right eye have to be created separately.
3. Description of Related Art
Displays with spatial light modulators in conventional LCD technology may, for example, be used for encoding and reconstructing. Known transmissive flat displays with high resolution may be used for large-area reconstructions.
Light modulators with cells which directly modulate the phase of the light may preferably be used, such as light modulators based on Freedericksz cells. However, this invention can also be applied to other spatial light modulators.
An illumination system for a computer-generated hologram, where the representation of a vertical parallax in the reconstruction is disregarded, is disclosed in WO 03/021363. The illumination system uses a line light source composed of conventional point light sources. This line light source emits collimated light. It is disposed in the focal plane of a cylindrical lens arranged at a right angle to it and creates a multitude of plane waves, which illuminate a SLM in transmission mode at various angles of incidence. In contrast to a point light source, the image is thereby uniformly illuminated without a diffuser being needed.
Document WO 00/75699 discloses a holographic display which reconstructs a video hologram with the help of partial holograms. Partial holograms being encoded on a common electronically addressable spatial light modulator (EASLM) are sequentially projected to an intermediate plane. This process is executed fast enough for an observer to perceive the reconstructions of all partial holograms as a single reconstruction of the entire 3D object.
The partial holograms are arranged in a regular structure in the intermediate plane by a specially designed illumination and projection system, for example including a shutter which is controlled in synchronism with the EASLM and which only allows the corresponding partial hologram to pass through and which in particular blanks diffraction orders that are not used. Difficulties occur when realizing an illumination system to illuminate each partial hologram with the required coherence and under the correct reconstruction angle. In order to avoid a large lens to be used as the optical element for reconstruction, it has been proposed to use a lens array.
WO 2004/044659 (US2006/0055994) filed by the applicant also describes a device for reconstructing three-dimensional scenes by way of diffraction of sufficiently coherent light, which contains a point light source or line light source, a lens for focusing the light and a spatial light modulator. In contrast to conventional holographic displays, the SLM in transmission mode reconstructs a 3D-scene in at least one virtual observer window. Each observer window is situated near the observer's eyes and is restricted in size so that the observer windows are situated in a single diffraction order, so that each eye sees the complete reconstruction of the three-dimensional scene in a frustum-shaped reconstruction space, which stretches between the SLM surface and the observer window. To allow a holographic reconstruction free of disturbance, the observer window size must not exceed the periodicity interval of one diffraction order of the reconstruction. However, it is at least large enough to see the entire reconstruction of the 3D-scene through the window(s). The other eye can see through the same observer window or is assigned a second observer window, which is accordingly created by a second light source. If the positions of the observer's eyes change, a tracking system displaces the light sources and thus tracks the observer windows accordingly. Here, a visibility region, which would typically be rather large, is limited to the locally positioned observer windows. The known solution reconstructs in a diminutive fashion the large area resulting from a high resolution of a conventional SLM surface, reducing it to the size of the observer windows. This leads to the effect that the diffraction angles, which are small due to geometrical reasons, and the resolution of current generation SLMs are sufficient to achieve a high-quality real-time holographic reconstruction using reasonable, consumer level computing equipment.
However, the known solution exhibits the disadvantage that a large, voluminous, heavy and thus expensive lens is required for focusing due to the large SLM surface area. Consequently, the device will have a large depth and weight. Another disadvantage is represented by the fact that the reconstruction quality is reduced significantly due to aberrations at the margins when using such large lenses.
A further disadvantage of the known embodiments is an insufficient luminous intensity of the SLM. Current solutions show a luminous intensity of the order of 1 cd/m2 and are hence far below the intensity of a conventional display (ca. 100 cd/m2). One reason for the low brightness is the low intensity of the coherent light sources on the SLM.